1. Field of the Invention
The invention relates in general to elevator systems, and more specifically to elevator systems of the traction type.
2. Description of the Prior Art
Elevator systems of the traction type which operate at speeds above about 500 feet per minute require a car speed feedback signal to determine the deviation of actual car speed from the desired car speed, and to use the deviation to take the corrective action necessary to closely regulate the car speed to the desired speed pattern. Signals are also provided when the elevator car passes certain relatively low speed values as it accelerates and decelerates, in order to determine when certain control functions should be performed, as well as to monitor the operation of the elevator car at predetermined points, such as during slow down and leveling. A car speed checking arrangement is disposed adjacent each travel limit of the elevator car, in order to determine if the car is slowing down within prescribed limits, and if it is not, to provide auxiliary terminal slow down means. The speed control for the elevator car is stabilized with a stabilizing feedback signal related to the rate of change of car speed. The stabilizing signal should not introduce low frequency electrical noise into the control signal to which the car is capable of responding.
These car speed related signals should be generated as accurately as possible, and with as little electrical noise in the signals as possible, in order to reduce stability problems. Further, in order to reduce system cost without sacrificing reliability the signals should be generated by low cost apparatus in a self-checking, fail-safe manner.
In the prior art, it is common to utilize a tachometer belted to the drive motor for developing the car speed feedback signal. The belt, gear teeth and eccentric gears, as well as the slots, commutator bars and brushes used in the construction of the tachometer, all add electrical noise to the velocity signal, but it is a reliable arrangement and broken belt switches make it safe.
The car speed indicating signals which indicate whether or not the elevator car is above or below predetermined relatively low speeds may be generated by speed switches operated in accordance with the speed of the drive motor, such as the magnetically coupled car speed responsive sensor disclosed in U.S. Pat. No. 3,802,274, which is assigned to the same assignee as the present application. These sensors are belt driven from the elevator drive motor, and while the speed points are sometimes difficult to set, and there is hysteresis between the operating points of the switches during acceleration and deceleration, the switches are rugged and reliable and safe because of broken belt switches.
The car speed checking arrangement adjacent the terminals or travel limits of the elevator car may monitor the floor selector, and if the floor selector is not operating in a manner which will produce a normal slow down, an auxiliary speed pattern is produced for controlling terminal slow down. In one prior art arrangement with an electromechanical floor selector a long cam disposed adjacent each terminal opens a series of switches mounted on the elevator car, one after another, and if the floor selector is operating properly, for each cam operated switch opening in the hoistway there should be a switch closing on the floor selector carriage. If this fails to occur, the auxiliary speed pattern is provided. U.S. Pat. No. 3,779,346, which is assigned to the same assignee as the present application, develops a terminal slow down arrangement which may be used with a solid state form of floor selector wherein spaced teeth adjacent each terminal cooperate with a sensor disposed on the car to detect overspeed and to automatically provide the correct slow down pattern if necessary. This arrangement operates with a low inertia, fast acting car speed sensor switch as a backup, such as the speed sensor disclosed in U.S. Pat. No. 3,814,216, which is assigned to the same assignee as the present application.
The stabilization signal may be obtained by taking the derivative of the drive motor armature voltage, or the derivative of the counter e.m.f. developed by the armature of the drive motor, when a direct metallic connection to the motor armature circuit can be tolerated. When a direct connection is not practical, such as in an elevator drive system with a solid state source of electrical potential for the drive motor, instead of a rotating source, the magnetically coupled acceleration transducer disclosed in U.S. Pat. No. 3,749,204, which is assigned to the same assignee as the present application, may be used. This arrangement provides a stabilizing signal responsive to the rate of change of the motor counter e.m.f.
Thus, in a single elevator system, many different types of apparatus may be used to generate the various speed responsive signals necessary in order to efficiently and safely control the operation of an elevator car. It would be desirable to reduce the amount and cost of the apparatus required to generate these speed related signals, if such reduction of apparatus and cost can be accomplished while maintaining the reliability and fail-safe characteristics of the prior art system arrangements.